1. Field of the Invention
The present invention relates to optical systems and optical transformers, and more particularly to systems that re-orient axes of light beams, such as emitted by laser diodes, and combine the light beams in order to improve brightness of combined beams.
2. Description of Related Art
The light output from semiconductor diode lasers is often required to be focused into an optical fiber or similar small spot for applications including the optical pumping of solid-state laser crystals. For these applications and others, it may be desirable to combine the output beams from a plurality of diode lasers onto a single spot of dimensions similar to those obtained when focusing the light beam from a single diode laser. The ability to achieve this is limited by the so-called brightness of the laser diode source; for the purpose of this discussion, brightness refers to the amount of optical power emitted by a source per unit solid angle and per unit cross sectional area. It is well known from optical theory that, by combining the illumination from several individual and identical sources through the use of mirrors, lenses or other passive optical components, it is impossible to increase the brightness at a remote position beyond the brightness of an individual source. In practice, this optimal result is not achievable; the resultant remote illumination formed by the summation of light from the discrete sources is usually much less bright than any of the individual sources. The total optical power at the remote position may be nearly the sum of the individual sources, but the illumination may be spread out over a cross-sectional area much greater than the sum of the emitting areas or the light may be distributed into an excessively large solid angle. For example, the large angular distribution may preclude the coupling of the light from the combined sources into an optical fiber of small angular acceptance, even though the resultant source may be smaller in spatial extent than the input dimensions of the fiber.
The light emitted from a laser diode is characterized by having a brightness that varies greatly when measured in two distinct and perpendicular planes. Specifically, for a typical laser diode with an elliptically shaped output beam produced by an emitting region measuring several hundred microns by a few microns, and oriented such that the axis defining the wider dimension is aligned horizontally, the light emitted vertically may be considered to be many times brighter than that emitted in the horizontal direction. The concept of etendu, the product of the angular and spatial extent of a source in two perpendicular planes, and which is inversely proportional to brightness, is hereinafter used to describe the problem of optical beam combining. For purposes of illustration in FIG. 1, a system of coordinates 1 is defined, with the x-z plane arbitrarily defined as horizontal and the y direction defined as vertical, a typical laser diode 2 is considered, having an emission 3 with a height xcex94y of 1 xcexcm, with an angular divergence xcex8yz of 65 degrees in the vertical direction, a width xcex94x of 200 xcexcm, and with an angular divergence xcex8xz of 14 degrees in the horizontal direction. With the definition of numerical aperture N.A.=sin (2 angular divergence), the etendu in the vertical direction is then the product of the vertical beam width and the vertical N.A. and is then 0.54 xcexcm*N.A. Similarly, the etendu in the horizontal direction is 24 xcexcm*N.A.
Since the angular divergence of the laser diode emission is substantially different in the x-z and y-z planes, the illumination profile, or spatial extent of the diode laser emission changes rapidly with distance from the emitting surface of the diode. Said profile, described by ellipses 4a-d, denotes the extent of the laser diode emission where the illumination intensity has fallen to a value of one half that at the location of peak intensity. Such elliptical profiles are used throughout the figures in the following descriptions of prior and subject art to denote the illumination profile of the diode laser beam as it progresses through various transforming components.
Fan (U.S. Pat. No. 5,081,637) has shown that a nearly optimal way to combine the light from discrete sources with radically different etendu in two perpendicular axes is to physically orient the diodes in such a way that the low etendu axes of the diodes are aligned to be collinear with respect to each other, with the high etendu axes aligned perpendicular to the common low etendu axis. This embodiment is described in FIGS. 2 and 3. In FIG. 2, the light from an individual source 2, propagating nominally in a z direction as defined by the coordinate axes 1, is first collimated in the low etendu direction by a simple cylindrical lens 5, forming a diode-lens combination 6. As shown by the elliptical intensity profiles 7a and 7b before and after the influence of the cylindrical lens 5, the effect of said lens is to substantially reduce the angular divergence of the diode laser beam in the y-z plane. In FIG. 3, a plurality of said diode-lens combinations 6a-c, three of which are shown, are then uniformly distributed and followed by an additional lens 8 that collects and focuses the light from the individual collimated sources to a remote image position 9. With high-quality collimating lenses and accurate positioning of the components, it is possible to achieve a brightness at the remote image position 9 nearly equal to that of an individual source 3a-c while simultaneously increasing the total optical power incident at the remote image position.
In the embodiment of Fan (U.S. Pat. No. 5,081,637), the emitting surfaces of the individual diodes are generally coplanar and the directions of light propagation from the multiple sources are essentially parallel, while in the technique described by Streifer (U.S. Pat. No. 4,826,269), the individual sources may be oriented as if on the surface of a cylinder, with the individual propagation directions aligned to be coplanar but generally pointing toward a common point of intersection.
The objective of the subject invention is the efficient combining of the illumination produced by the individual laser sources within a laser diode array. Laser diode arrays consist of a plurality of discrete laser diode sources fabricated onto a common semiconductor wafer, and said arrays are powerful sources of optical radiation. Each discrete diode source has an elliptical intensity profile as described earlier in FIG. 1, and the plurality of said sources is aligned such that the high etendu axes of all sources are collinear and their emitting faces are coplanar. Referring to FIG. 4, the array 10 has a light output with a very high etendu in the axis 11 common to all sources 12a-e and a low etendu in the y-direction perpendicular to said common axis. The centers of each source region may be separated by a distance w that varies between 1.5 and 5 times the width xcex94x of a single source in the high etendu direction, and the array may consist of between 10 to 60 individual sources, distributed uniformly over a length of 0.5 to 2 centimeters. Because of the combined effects of the individual source spacings and orientations, the brightness of the array taken as a whole is much lower than the brightness of an individual source in the array. It is desired to combine the light from these discrete sources into a single spot, yet it can be see from the work of Fan (U.S. Pat. No. 5,081,637) that these individual sources are oriented improperly to allow for a straightforward combining of the light from the individual sources into a single spot while minimizing the etendu at the remote source. Specifically, the high-etendu axes 11 of the sources comprising the array are collinear, whereas the desired orientation is the collinear alignment of the low-etendu axes 13a-e. If the sources 12a-e comprising the array could be physically separated and rotated 90 degrees, the techniques of Fan (U.S. Pat. No. 5,081,637) could be utilized. Rather than an actual physical rotation of the individual sources, it is the objective of the subject invention to optically realign the light emitted from the individual sources to meet the criterion shown by Fan (U.S. Pat. No. 5,081,637) to allow for a combining of the optical output of the individual sources while maintaining high brightness of the remote source.
Several authors have described prior art to solve this problem. For example, the work of Endriz (U.S. Pat. No. 5,168,401) describes a technique utilizing a micro-optic element placed in front of each source to perform the aforementioned rotation, followed by appropriate focusing lenses. The disadvantage of the prior art of Endriz (U.S. Pat. No. 5,168,401) is the need for micro machined mirrored surfaces requiring special and expensive manufacturing techniques.
The subsequent work of Spaeth (U.S. Pat. No. 5,808,323) et al. describes how the individual sets of mirrors may be fabricated from identical and parallel mirror sheets, aligned and assembled with relatively inexpensive conventional optical assembly techniques. However, a disadvantage of the prior art of Spaeth (U.S. Pat. No. 5,808,323) is that, as the light from each source diverges in a plane common to all sources, some fraction of this light may strike the sides of the adjacent mirror plates, thereby scattering the light into a direction not recoverable and focusable, further resulting in said fraction of the optical power emitted by the laser array being lost.
The prior art of Lissotschenko (WO 99/46627) describes how the reorientation process may be achieved by a substantially different technique. This approach, which may be referred to as the Fourier transform technique, separates the reorientation process into several steps, each of said steps physically separated from another. Each step is characterized by a transformation between positional and angular information and the devices used at each step may be referred to as transformers. Referring to FIG. 5, the operation of this technique is illustrated by imposing a set of coordinate axes 1 onto the optical system, with the x axis aligned horizontally through the center of each emitting source, the z axis aligned parallel to the common propagation direction of the light from each source 14a-e and the y axis oriented vertically and perpendicular to the x and z axes. The optical sources 14a-e used with this technique are the individual laser diode sources within an array of multiple sources, with an additional arrangement of collimating lenses to reduce the horizontal and vertical angular divergence of the light output of each of said individual sources, one of said lens/source combinations being shown in FIG. 2. These lensing components and individual laser sources are then ignored to simplify the figures. However, it is noted that, while the collimation of the illumination dispersed vertically is readily achieved with a single cylindrical element, the collimation of the illumination dispersed horizontally requires a separate lens for each diode laser within the plurality of diode lasers comprising the diode laser array.
Three basic transformations are combined to form the complete system of the Lissotschenko patent (WO 99/46627) and are schematically displayed in FIG. 5. The coordinate system 1 will be used as a reference for defining a given direction or orientation. For the purposes of illustration, 5 separate sources 14a-e are considered, although the number of sources is, in principal, only limited by the ability to fabricate appropriate optical components to process the illumination from said sources. The shape or transverse distribution of each light beam as it propagates through the optical system is denoted by a series of ellipses 15-18 centered along and placed perpendicular to the direction of propagation of the light from each source.
In the first transformation, the propagation directions 19a-e of the light from the sources are redirected upward or downward from the horizontal or x-z plane at an angle y whose tangent is directly proportional to the position of each source measured from the center of the array at x=0. Sources at x greater than 0 are angularly directed increasingly upwards while sources at x less than 0 are angularly directed increasingly downwards. As an example, the emission from a source at the extreme left side of the array at a position x=xe2x88x92a is redirected downwards at an angle xe2x88x92xcex3 while the light from a source at the extreme right side of the array at position x=+a is redirected upwards at an equal and opposite angle+xcex3.
The second transformation is equivalent to the well-known Fourier transform which maps a distribution of angular deviations in the vertical direction to a distribution of spatial positions also in the vertical direction. Similarly, the Fourier transform maps the input spatial distribution in the horizontal direction to an output angular distribution in the horizontal direction. Therefore, the light from a source 14a at x=xe2x88x92a is repositioned to a location (x,y)=0,xe2x88x92b while the light from the source 14e at x=+a is repositioned to a location (x,y)=0,b. Similarly, the light from the source 14e now redirected to (x,y)=0,b propagates at an angle +xcex2 with respect to the y-z plane, while the light from the source 14a redirected to (x,y)=0,xe2x88x92b propagates at an equal and opposite angle xe2x88x92xcex2 with respect to the y-z plane. It can be appreciated that the sources 14a-e distributed along the x-direction at the first Transformation have been transformed to a set of virtual sources 17a-e distributed along the y direction at the third transformation.
Approaching the position of the third transformation, the light beams have undesirable angular displacements xcex2 in their respective propagation directions that increase with their distances above and below the x-z plane. A third transformation removes the undesirable angular deviation from each light beam, resulting in the illumination from all sources directed parallel to the z axis. Subsequently, the illumination from the multiple sources may be focused to a point 20 in the manner described by Fan (U.S. Pat. No. 5,081,637) by using a single spherical lens or some combination of spherical and cylindrical lenses.
Next, the transformers responsible for the aforementioned first, second and third transformations are described in FIG. 6. In the prior art of Lissotschenko (WO 99/46627), the first transformer 21 is a refractive element with a first planar input surface 21a and a second opposing output surface 21b which makes an angle xcfx86 with said input surface. Further, the first transformer is constructed so that said angle separating the first input surface and second output surface changes continuously with position x progressing from one end of the diode array to the other. The shape of the second surface 21b may be said to have the shape of a twisted ribbon or propeller.
Referring to FIG. 7, to achieve the desired transformation properties described earlier and to achieve the final optical reorientation of the sources as described by Fan (U.S. Pat. No. 5,081,637), it is required that the horizontal extent t of the optical illumination at the first transformer not be an appreciable fraction of the spacing w between the centers of said sources. It can be seen that, in the invention of Lissotschenko (WO 99/46627), an undesirable rotation xcexa3 is induced in the orientation of the high-etendu axis at the first transformer, caused by the continuous change in angle between said first surface 21a and said second surface 21b of the first transformer 21. Since in actual practice, the space 22 between the optical source and the first transformer 21 is often occupied by one or more optical elements intended to collimate the illumination in the vertical direction, the horizontal extent of the light has often expanded to an undesirable size at first transformer. When taken in combination with the induced rotation xcexa3 in the spatial illumination profile, this causes a diminishment in the efficient eventual recombination of the light from the individual sources. Further, fabrication of the twisted surface 21b of said first transformer requires sophisticated optical fabrication techniques, such as those described in the prior art of Lissotschenko (WO99/46627), not commonly used in conventional optical fabrication facilities.
Returning again to FIG. 6, it can be seen that the second transformer 23, a conventional plano-convex, bi-convex or Fresnel lens, is positioned such that the first transformer and third transformer are placed one focal length f away and on opposite sides of said lens, as is well-know within the art, thereby performing the Fourier Transform described earlier.
The third transformer 24 as described by Lissotschenko (WO 99/46627) consists of a first input surface 24a, followed by a series of second refractive surfaces disposed along the x axis, only one of which 24b is shown in FIG. 6, each being of a width 25 equal to a small portion of the total width of the third transformer, and with each of these individual second vertically aligned surfaces oriented at an angle xcexc with respect to said first input surface. Further, the angle xcexc subtended between said first and second surfaces varies continuously with position in the y or vertical direction so as to enable the previously-described third Transformation of the incident optical beams. While it is desirable that said second surface be smooth and continuous across the full extent of the illumination incident along the x direction on said third transformer, the micromachining techniques required for the construction of the transformer with the embodiment of Lissotschenko (WO 99/46627) precludes the fabrication of surfaces with large changes in thickness. It is because of these limitations that the embodiment of Lissotschenko (WO 99/46627) requires that the third transformer take on the configuration of a series of segments or narrow stripes aligned side to side, each being of the shape described in FIG. 6. This segmentation of the second surface can result in increased scatter and optical losses.
It is an objective of the subject invention to perform the above described optical transformations without the need for expensive fabrication techniques such as those described in the inventions of Endriz (U.S. Pat. No. 5,168,401) or Lissotschenko (WO 99/46627).
It is a second objective of the subject invention to achieve the transformational properties of the optical devices described in the prior art by using conventional optical fabrication techniques as are commonly applied to the fabrication of flat, polished optical surfaces.
It is yet another objective of the subject invention to reduce the optical loss incurred in performing the optical reorientation as can occur in the step mirror embodiment of Spaeth (U.S. Pat. No. 5,808,323).
It is a further objective of the subject invention to describe a first transformer similar in function to that described in the embodiment of the Lissotschenko (WO 99/46627) that does not induce the adverse effect of optical rotation of the elliptical intensity profile about the direction of propagation.
It is yet another objective of the subject invention to describe an embodiment in which said first and third transformers are substantially identical.
It is yet another objective of the subject invention to describe a third transformer with a second surface that is smooth and continuous across the incident illumination originating from a given source.
The subject invention provides for an optical system for modifying the optical orientation of the individual sources of a laser diode array so that the illumination provided by said individual sources may be combined into a single target location in such a way as to substantially provide an illumination at said target with the brightness of the individual sources. To achieve this end, three optical devices, hereinafter referred to as optical transformers, provide for the angular and spatial repositioning of the light beams from said individual sources in a way that allows a subsequent lens or combination of lenses to focus the light from said repositioned beams to said single target. Specifically, a first set of uniformly-spaced parallel light beams, said parallel beams defining a horizontal plane, is repositioned into a second set of uniformly-spaced parallel light beams, also defining a second horizontal plane but propagating in a direction perpendicular to the original light beams, with said second horizontal plane offset from said first horizontal plane, while the relative orientation of the high-brightness and low-brightness axes of said light beams with respect to the horizontal is interchanged.
The first of said transformers consists of a series of identical and easily fabricated flat mirrors, placed parallel to each other and angularly oriented such that the optical beam from each of said sources is deflected into a uniquely determined angle measured with respect to a vertical plane aligned with the emitting faces of said sources, said angle determined by the relative position of said sources. The second transformer consists of a Fourier transform lens which projects a Fourier transformation of the angular and spatial distribution of light beams created by said first transformer, onto a third transformer. Said second transformer results in the spatial positioning of the optical beams according to their angular displacements set by the first transformer, while the angular displacements induced by said second transformer correspond to the spacing of said first set of uniformly-spaced parallel light beams.
The third transformer consists of a mirrored optical device which can be made substantially identical to said first transformer, and which removes the angular displacement of said linearly-dispersed optical beams induced by said Fourier transform lens, resulting in a series of parallel light beams dispersed horizontally, with the high-brightness axis of the elliptical intensity profile of each said beam aligned coaxially. This distribution of light beams may then be focused into an optical fiber, laser gain medium or other device requiring the brightness-conserving coupling of individual optical sources into a single target location.